This disclosure sets forth methods and apparatus for measuring pollutants contaminating earth formations. More particularly, the invention involves the three dimensional mapping of the resistivity of near surface volumes of earth formation, the premise being that pollutants such as hydrocarbons and brine usually exhibit a significantly higher resistivity and lower resistivity, respectively, than most virgin earth formations. By mapping three dimensional resistivity contours of the site under investigation, the location, magnitude and migration of pollutants can be quantitatively determined.
The pollution of near-surface earth formations is a problem drawing world wide concern. Leaking underground gasoline, fuel oil and other hydrocarbon storage tanks are a major source of pollution of potable ground waters. Likewise, brine used in borehole drilling operations and stored in surface and underground storage containers is also a major ground water pollutant. Another source of pollution is natural saline water from deep formations which often migrates toward shallow potable ground water formations, the initial path of migration being around oil and gas well casing which has not been properly cemented to obtain the desired zonal isolation.
The detection, quantification and monitoring of pollutants near the surface of the earth has historically been an expensive and time consuming operation. Traditional techniques involve the drilling of a multiplicity of shallow boreholes throughout the area of concern, retrieving of core samples from the drilling operation, and analyzing the core samples for pollutants. This technique will not detect any polluted areas which have not actually penetrated by a borehole. A second technique involves the drilling of a multiplicity of monitor wells throughout the area of concern. Sensors are temporarily or permanently placed at varying depths within the monitor wells, and various parameters indicative of the pollutants are measured as a function of time. The basic premise of this technique is that either the boreholes penetrate the pollutants or the pollutants will migrate with time to the vicinity of one or more boreholes thereby being detected by the borehole sensors. Again, this technique is very expensive and time consuming, especially if detection requires the pollutants to migrate to the vicinity of the borehole before being detected.
It is highly desirable to obtain a spatially continuous, three dimensional measurement of possible pollutants in the area of concern. Pollutants can and often are initially isolated within small volumes or traps, only to migrate in time over a larger area. There is a high probability that discrete sampling techniques such as well bore core sampling or even monitor well measurements over time will not detect pollutants until they have contaminated an extensive volume of earth formation. At this point, remedial clean up is extremely costly and significant environmental damage has already occurred.
Surface transmitters and well borehole receivers have been used in the past to determine resistivity of intervening earth formation. U.S. Pat. No. 2,746,009 to McLaughlin et al. discloses a method directed to locating ore bodies. Main and vernier transmitter coils are set at one location at the surface and a receiver is conveyed along a nearby bore hole. The receiver coil detects primary signal and any secondary signal resulting from a low resistivity ore body anomaly. The vernier coil is rotated about its axis giving measurements which are related to the direction of an ore body in a plane comprising the main transmitted, the vernier transmitter, the downhole receiver, and the ore body. The method of McLaughlin et al. does not, however, yield a three dimensional map of resistivity anomalies. Ruehie discloses in U.S. Pat. No. 3,391,334 the use of a surface transmitter and three vertically spaced borehole receivers to measure resistivity of a layer of earth formation bounded by the vertical extent of the receiver array. A three dimensional subterranean map of resistivity cannot be obtained using the method disclosed by Ruehie.
U.S. Pat. Nos. 4,323,848 and 4,502,010 to Kuckes discloses means of measuring the resistivity of earth formations using a surface transmitter and a downhole receiver comprising a magnetometer. The thrust of both inventions is directed to improvements of the magnetometer receiver. Two embodiments of the surface transmitter are disclosed.
Spatially continuous three dimensional measurements of resistivity cannot be obtained using the means or methods taught by Kuckes. The first embodiment comprises a circular antenna concentric with the top of the wellbore with the axis of the loop being perpendicular to the plane of the surface of the earth. The second embodiment comprises two electrodes inserted in to the earth surface on opposite sides of the top of the borehole. In both embodiments, the transmitter remains fixed with respect to the position of the borehole.
U.S. Pat. Nos. 4,748,415; 4,901,023; 5,065,100 and 5,260,661 to Vail, III are directed toward measuring the resistivity of earth formation from within a borehole cased with highly conducting material such as steel. Surface transmitters are employed as well as downhole receivers. In one embodiment, two surface transmitters comprising circular antennas are used with the first being concentric with the top of the borehole and the second being positioned remote from the borehole with the axis being perpendicular to the plane of the earth's surface. The transmitters remain fixed with respect to the position of the borehole. The frequencies are in the 0.001 to 20 Hz range. Low frequencies such as these are needed to penetrate the highly conducting borehole casing. Resolution, being related to frequency, is correspondingly low. As with all of the prior cited patents, spatially continuous measurements in three dimensions are not obtainable using the disclosures of Vail, III.